1. The present invention relates to magnetic memory cells used to store digital electronic data and, more particularly, to an improved magnetic memory cell which functions as a random access memory.
2. Description Of Related Art
The dipole magnetic moments of neighboring atoms within a small region, or domain, of a thin film of magnetic material align themselves when placed in a sufficiently strong external magnetic field. This alignment of magnetic dipole moments is unique to magnetic materials (such as Fe, Co, Ni, Gd and Dy) and takes place despite the random motion generally undergone by atoms within any material. The orientation of the magnetic dipole moments remains after the external magnetic field is removed.
Transition regions exist between any two domains which do not have the same alignment of magnetic dipoles. The transition regions between such domains are called domain walls. Different types of domain walls typically exist in magnetic material, each unique with respect to the orientation of the magnetic field existing within or comprising the domain wall. Three types of domain walls are Nel walls, cross ties, and Bloch lines or walls. Within a Nel wall the magnetic field rotates in the plane of the thin film, and the Nel wall separates two antiparallel domains in the film. Reversing the magnetic field direction in a small portion of a Nel wall results in the creation of a cross tie. The cross tie magnetization generally opposes the magnetization in the domains separated by the Nel wall. Within a Bloch line the magnetic field rotates out of the plane of the thin film. The magnetization associated with a Bloch line generally parallels the magnetization in the domains separated by the Nel wall.
As briefly discussed above, associated with each cross tie is a section of the Nel wall that is reversed or inverted. That portion of the Nel wall is often referred to as a negative Nel wall with the non-inverted Nel wall being referred to as the positive, stable, or dominant Nel wall. The negative Nel wall section is bordered by a cross tie on one end and a Bloch line on the other end.
The characteristic magnetic fields of the domain wall types remain unchanged in the absence of an external magnetic field of a predetermined strength. In the presence of an external field of the predetermined strength, however, the magnetic state of a domain wall, or domain state, at any given location in the thin film can be changed.
The stable magnetic domain states of the magnetic film represented by the domain wall magnetization fields may be utilized within a memory system for the storage of digital data. Such a memory system is referred to as a "cross tie" memory. In a binary cross tie memory system the stable Nel wall and the alternative stable reversed Nel wall with a cross tie and Bloch line pair can be used to represent bits of data. The data can be written into memory by the application of an appropriate magnetic field Where a positive Nel wall exists, a cross tie/Bloch line pair can be introduced by the application of the appropriate magnetic field and will represent a specific logic state (i.e., logic "1" or a logic "0"). An opposite field can be used to annihilate the cross tie and, thus, restore the original Nel wall domain state. That stable state will represent the opposite logic state (i.e., a logic "0"0 or a logic "1"). For the purpose of this description, a logic "1" will be represented by the presence of a cross tie, and a logic "0" is the presence of only a positive Nel wall.
The data can also be read from the magnetic memory. The read-out may be accomplished by the use of magneto-resistive effects The introduction of small magnetic fields into the domain walls of the magnetic film changes the resistance within the film. That resistance change can be measured and varies according to the domain state of the magnetic film. The resistance change is small if the domain state is a Nel wall, and the resistance change is larger if the cross tie state exists. A measurement of the resistance reveals the state of the domain wall and, thus, the logic state which represents a digit or bit of stored data. The precise amount of the resistance measured in each state and the measured resistance change differs according to which cross tie memory system is being practiced.
Reading and writing of individual domains within the magnetic film of such a system is accomplished by means of conductors aligned over positioned domain walls. The currents through the conductors create the small and larger magnetic fields which allow the magneto-resistive reading and the writing of data, respectively.
U.S. Pat. No. 3,868,659, issued on Feb. 25, 1975 to Schwee, discusses the use of thin film magnetic materials as data storage devices. A more recent disclosure of such use of thin film magnetic materials is contained in U.S. Pat. No. 4,246,647, which issued on Jan. 20, 1981 to Johnson et al. In both of those patents, the memory disclosed is a serial memory, i.e., once a data bit is entered at one end of the memory, it is passed through the memory and cannot be removed until all data entered ahead of it has been removed. The operation of such a memory is described in the above-referenced patents. Those serial memories have obvious limitations in that it is often desirable to randomly access data which has been stored in the memory. In a serial access memory, to access a given data bit, it is necessary to first read out all data which was entered before the data bit of interest can be read.
To overcome the problems and disadvantages of the prior art serial access memories, random access magnetic thin film memory systems have been proposed within which each memory element can be randomly accessed to either read or change the data bit stored in it. One such thin film random access memory ("RAM") is described in U.S. patent application Ser. No. 933,516 filed Nov. 21, 1986, in the name of John F. Jackson, and assigned to the assignee of this invention. Another thin film RAM is described in U.S. patent application Ser. No. 933,709, filed Nov. 21, 1986 in the name of Elizabeth H. Ginnes and Charles W. Baugh and also assigned to the assignee of this invention.
A portion of one cross tie random access memory is shown in FIG. 1. Cross tie random access memory 5 includes substrate 6 with strips 7, 8 of thin film magneto-resistive material. Spaced memory elements 9, 10, 11, 12 are contained in the thin film in an array. Memory elements 9, 10, and 11, 12 disposed on thin film strips 7, 8, respectively, may be pictured as comprising vertical columns m.sub.1, m.sub.2 of spaced memory locations. Similarly, memory elements 9, 11 or 10, 12 may be seen as forming horizontal rows n.sub.1, n.sub.2 of spaced memory locations.
Metallic row conductors 13 and 14 overlay memory elements 9, 11, and 10, 12 respectively; while column conductors 15 and 16 overlay memory elements 9, 10 and 11, 12 respectively. Write module 17 is connected to both row conductors 13, 14 and column conductors 15, 16. Read module 18 is connected to row conductors 13, 14 and to thin film strips 7, 8.
In operation, the individual memory elements are initialized, or aligned, by placing a sufficiently large magnetic field perpendicular to the columns of memory elements. The field initially aligns the magnetic moments in the thin film and creates a positive Nel wall in each memory element 9, 10, 11, 12. After initialization, data can then be written into RAM 5 of FIG. 1 as follows. First, the process circuitry of the memory system determines that a data bit should be placed in a specific memory element. For writing in a data bit, write module 17 then generates a potential of predetermined value and polarity which is applied to both the row conductor and column conductor connected to the selected or addressed memory element to change its and only its magnetic state. The memory element written into is that location where both the overlaying column and row conductor have been supplied with the potential of predetermined value. For example, if a data bit is to be written into memory element 9, write module 17 will generate a sufficient potential on selected row conductor 13 and selected column conductor 15. The current through the selected row conductor must be insufficient by itself to generate a magnetic field strong enough to change the magnetic state of any of the memory elements in the row (e.g., elements 9 and 11). Similarly, the current through the selected column conductor must be insufficient by itself to change the magnetic state of the memory elements in the column (e.g., elements 9 and 10). Thus, in the memory elements where both the overlying row and column conductors of RAM 5 have not been coincidently supplied with current (i.e., the unaddressed memory elements) no stable magnetic state should change. However, the addressed memory element is subject to the magnetic fields generated by both the selected row conductor and the selected column conductor. There the magnetic field is large enough to change the magnetic state of that addressed memory element and, thus, the data bit is written in as desired. By changing the polarity of the potential applied to the selected row and column, the direction of the currents in the selected conductors can be changed and the generated magnetic fields reversed to write either a logic "1" or a logic "0" into the addressed memory element.
To read out a data bit from an addressed memory element, read module 18 generates a potential having a predetermined value and polarity which is applied to a row conductor 13 or 14 in which the addressed memory element is located. The applied potential generates a current which, in turn, generates a magnetic field in the selected row. The field places the addressed memory in a condition of changed resistance. The generated magnetic field, although sufficient to change the resistance of the memory elements, must be small enough not to cause a change in the stable magnetic states of any of the memory elements.
The resistance change undergone by a memory element depends on the magnetic state of the element. The different resistance changes permit a distinction between the stored states at different memory element locations and, therefore, allows the data bit stored in a memory element to be read. For example, if a data bit is to be read from memory element 9 of RAM 5 in FIG. 1, read module 18 will select row conductor 13 and thin film strip 7, will apply the necessary potential to row conductor 13, and will sense the current change in strip 7 which indicates the change in resistance of addressed memory element 9. The resistance change is determined by using conventional sensing techniques.
In the above described cross tie RAMs, current flows past, and thus magnetic fields are generated near, not only the addressed memory element but also other memory elements in the array. For example, if memory element 9 is the element addressed for the writing in of a data bit, current flows through both row conductor 13 and column conductor 15. Thus, not only are magnetic fields generated at addressed element 9 as desired but also a magnetic field is generated at unselected memory element 11 due to the current in row conductor 13 and at unselected memory element 10 due to the current in column conductor 15.
Similarly, in the read mode, current flows past, and thus magnetic fields are generated near, not only the addressed memory element but also other memory elements in the row in which the addressed memory element is located.
Such a situation places design restrictions on the operating margins of the memory. Specifically, the magnitude of the current that can be used in conductors must be carefully selected and controlled. Since each memory element may have a different threshold level at which it will change its stable magnetic state, the current in the rows and conductors must be at least half as small as the smallest threshold switching current of the most sensitive memory element in the array. Otherwise, the stable magnetic state of unaddressed memory elements may be changed unintentionally, thus, causing data bit errors in the cross tie RAM.
The use of coincident currents to write a data bit into an addressed memory element causes potential unacceptable bit errors in another manner. The generation of magnetic fields at the unselected memory elements, makes those memory elements much more susceptible to undesired stable magnetic state switching, such as from the influences of magnetic fields external to the memory system.
As can be seen from the above discussion, cross tie RAMs in which the reading and writing of data is accomplished in such a manner that magnetic fields are generated in unaddressed memory elements as well as in the addressed memory element, there is a potential for unacceptable bit error. Thus, from the foregoing consideration, it should be apparent that there is a great need for an improved cross tie RAM in which the problem of unacceptable bit error is alleviated.
It is, thus, intended that the invention provide a magnetic thin film random access memory system in which there is less chance for data bit error.
Another intent is that the invention provide a magnetic thin film random access memory system in which the reading in and writing out of data is accomplished by current flowing only under the one memory location which is being addressed.
Still another intent is that the invention provide a magnetic thin film random access memory system in which a large current may be used to write a bit into an addressed memory element without affecting the other memory elements of the memory array.
Yet another intent is that the invention provide a magnetic thin film random access memory system in which the effects of ambient electromagnetic fields is minimized.
A further intent is that the invention provide a magnetic thin film random access memory having high operating speed and high density, is non-volatile, and is operable over a wide operating temperature range.
Other intentions and features of the invention will further become apparent with reference to the accompanying drawings and the detailed description of the invention or may be learned by practice of the invention.